1. Field of The Invention
Applicants"" invention relates to devices that serve as antennas to receive and transmit electromagnetic energy. More specifically, the present invention relates to the use of antennas for direction-finding systems, particularly in the high frequency band of operation. The antennas are designed and fabricated to be conformable to flat and semi-flat surfaces and to have low levels of radar reflections. These characteristics provide certain advantages, particularly with respect to their being more difficult to observe or detect, especially by radar.
2. Background Information
Presently, antennas are used in many configurations and with many different electrical connections to receive and transmit electromagnetic energy. Some antennas are very simple, such as antennas for car radios, but other antennas have large numbers of antenna elements in complex geometric arrays with very complex electrical connections between the elements. A common feature of all antennas is that they convert electrical energy into electromagnetic energy (transmit antennas) or electromagnetic energy into electrical energy (receive antennas).
Radio direction-finding (DF) is the process of determining the direction of arrival of a radio signal transmission. There are numerous direction-finding antennas and systems in the prior art.1 The techniques for obtaining bearings of an emitter and using triangulation to estimate target positions are well-known. The ability to ascertain the geographical location of an emitting transmitter offers important capabilities for many modern communications applicationsxe2x80x94such as land, air, and sea rescue, duress alarm and location, law enforcement, and military intelligence.
Some receiving antennas can be used for radio direction-finding purposes. There are a number of suitable types of antenna elements which can be positioned with respect to each other in different configurations. Examples of types of antenna elements include monopoles, dipoles, simple loops, and ferrite-loaded simple loops. Configurations include Adcocks, dipole Adcocks, quadrupole Adcocks, Rocke Adcocks, spaced loops, simple loops, Doppler arrays, and arbitrary arrangements used with vector-matching DF algorithms. Also, the antenna configuration can be rotating or non-rotating and fixed or mobile.
Typically, direction-finding antennas have been mounted high and/or on the external surfaces of platforms so they have unobstructed views of the arriving electromagnetic energy and are not near surfaces or objects from which the electromagnetic energy reflects or reradiates. This is especially desirable when the platform on which they are mounted is a ship, airplane, land vehicle, or building.
Herndon H. Jenkins, Small Aperture Radio Direction-Finding, Artech House, Inc., Norwood, Mass., 1991; Douglas N. Travers, xe2x80x9cEight Loop Antenna System and Method of Scanning Samexe2x80x9d, U.S. Pat. No. 3,329,954, issued Jul. 4, 1967; Terry C. Green and Ruell F. Solberg, Jr., xe2x80x9cFerrite Core Crossed Spaced Loop Antennaxe2x80x9d, U.S. Pat. No. 3,623,116, issued Nov. 23, 1971. 
Recent developments of computer capabilities, software, algorithms, and vector-matching DF techniques loosen the previous requirements of clean site responses from DF antennas. This allows antennas to be mounted in much less ideal locations. However, it is then more important that variability between antennas be reduced, so the direction-finding antenna characteristics not vary from antenna to antenna.
An inherent disadvantage of conventional direction-finding antennas is they have high reflections of radar signals. For certain conditions, such as during war, stealth or low observability characteristics are very important, and various techniques to reduce radar echoes have been used.2 Determining the radar cross section (RCS) of objects is another way to characterize and compare radar reflections from objects. RCS is a measure of the electromagnetic scattering from a target observed by radar. RCS is a function of the physical cross section area, shape, material, and orientation of the target and the frequency and polarization of the incident energy.3 
J. Wayne Burns, xe2x80x9cIntroduction to Stealth Technology and Stealth Aircraft Weight Penaltiesxe2x80x9d, SAWE Journal, Vol. 53, No. 1, Fall 1993, pp. 40-58; Roger A. Stonier, xe2x80x9cStealth Aircraft and Technology from World War II to the Gulf, Part I: History and Backgroundxe2x80x9d, SAMPE, Vol. 27, No. 4, July/August 1991, pp. 9-17; Roger A. Stonier, xe2x80x9cStealth Aircraft and Technology from World War II to the Gulf, Part II: Application and Designxe2x80x9d, SAMPE Journal, Vol. 27, No. 5, September/October 1991, pp 9-18; R. Neal Cain and Albert J. Corda, xe2x80x9cActive Radar Stealth Devicexe2x80x9d, U.S. Pat. No. 5,036,323, issued Jul. 30, 1991; Dwight L. Jaggard and Nader Engheta, xe2x80x9cNovel Shielding, Reflection, and Scattering Control Using Chiral Materialsxe2x80x9d, U.S. Pat. No. 5,099,242, issued Mar. 24, 1992; Gene P. Shumaker and Walter B. Mays, xe2x80x9cMulti-Fiber Species Artificial Dielectric Radar Absorbing Material and Method for Producing Samexe2x80x9d, U.S. Pat. No. 5,661,484, issued Aug. 26, 1997; Walter J. Dwyer, xe2x80x9cDished Annular, Radiofrequency Absorber and Method of Manufacturexe2x80x9d, U.S. Pat. No. 3,078,461, issued Feb. 19, 1963. 
W. M. Cady, M. B. Karlitz, and L. A. Turner, Radar Scanners and Radomes, McGraw-Hill Book Co., New York, 1984; Eugene F. Knott, John F. Schaeffer, and Michael T. Tuley, Radar Cross Section: Its Prediction, Measurement and Reduction, Artech House, Norwood, Massachusetts, 1985; George T. Ruck, ed., Radar Cross Section Handbook, Plenum Press, New York, Vol. 2, 1970, xe2x80x9cChapter 8. Complex Bodiesxe2x80x9d, (G. T. Ruck), pp. 539-670. 
Antennas that have much lower radar reflections, and much lower radar cross sections, have significant advantages when low observability is important. The present invention is a high-frequency (HF) antenna that has been designed to have much lower radar reflections. The HF range is defined to be 0.5 to 30 megahertz (MHz). The height of the present invention is on the order of a magnitude less than conventional high frequency antennas. The general profile area of the antenna assembly that radar transmissions would reflect from is much less. In addition, the surfaces that reflect radar energy are designed so the radar energy is reflected in directions that will not be received back at the site of the radar transmitter. These surface angles have been applied, for example, to the ferrite bar surfaces, the antenna base plate, and the antenna cover. Also, the intersections of the ferrite bars have corners at angles to reduce the radar reflections back to the transmitter. In addition, materials are used, such as for the antenna cover, that have inherently low radar reflectivity, and radar absorbing materials (RAM) can be used on the inside of the antenna cover and on surfaces of the ferrite bars and in the corners of their intersections with each other.
The present invention has other good low observable characteristics to make it difficult to detect by not only enemy radar, but also infrared and optical detection systems. No power is sent to the antenna assembly of the present invention. Since the antenna is passive, it does not generate heat and, therefore, does not have a large infrared signature. Since it has a low physical profile and can easily conform to surfaces of objects, its optical visibility is very low. The color of the antenna can be such that it matches the color of its platform. Also, it lends itself to being camouflaged.
Fighter and bomber airplanes that have stealth qualities have advanced significantly. The F-117A and the B-2 have physical characteristics that are distinguishable from other airplanes and are easily recognized visually. The advancement of surface ships for low observability or very low observability is still in the early stages. However, ships being developed with stealth qualities include the La Fayette stealth frigate in France, the Sea Wraith stealth corvette by Vosper Thornycroft in the United Kingdom, the Visby class of corvette in Sweden, and the DD 21 in the United States.
In general, these new surface ships have smooth, radar-defeating exterior shapes. Their radar dishes and antennas are typically enclosed within towers or other structures or they are blended into the ship structures.
It is an object of the present invention to provide a novel antenna which will conform to flat and nearly flat surfaces and to provide optimal direction-finding accuracy and sensitivity with low observability, especially with low radar reflectivity and low radar cross section characteristics.
It is another object of the present invention to provide a novel antenna with a much lower physical profile than existing antennas.
Still another object of the present invention is to provide a novel antenna that lends itself for use on ships, airplanes, buildings, and other platforms for which conformal shaping is a high priority.
It is another object of the present invention to provide a new and improved, high-frequency direction-finding antenna assembly.
Another object of the present invention is to provide a novel antenna for which satisfactory operation does not depend as critically on the placement location of the antenna on platforms as existing antennas do.
Yet another object of the present invention is to provide a novel antenna for use in wartime and during hostile conditions.
It is another object of the present invention to provide a novel antenna partially constructed of a ferrite material.
Still another object of the present invention is to provide a novel antenna that is economically manufactured, installed, maintained, and operated.
Another object of the present invention is to provide a novel antenna that is more resistant to environmental damage.
Yet another object of the present invention is to provide a novel antenna that is useful for search and rescue.
In satisfaction of these and related objectives, applicants"" present invention provides an antenna with these characteristics where the antenna is constructed of metal, ferrite, plastic, and composite materials. In times of military confrontations, this invention will be particularly useful to its practitioner. With the current use of large, metal antennas near the tops of platforms, radar reflectivity of the antenna is so high that adversaries can detect the practitioner""s ship from significant distances. In contrast, with a conformal, shaped antenna like that of the present invention, the radar reflectivity is reduced and an observer would have to be much closer before detecting the practitioner or the power of the radar would have to be significantly increased, which would, in turn, increase the detectability of the observer""s radar.